This application is a national stage of PCT/EP99/08433 filed Nov. 3, 1999 and based upon German national application 19852368.8 of Nov. 13, 1998 under the International Convention.
The invention relates to a marking device for application to objects and having a magnetic coding with areas of different magnetization.
For marking and thus individual correlation and securing of credit cards, access cards, electronic keys or the like, various magnetic codings are used, usually in the form of a so-called magnetic strip. For the coding, a terminal magnetic strip is magnetized selectively, i.e. areawise. The magnetic signature or coding can be detected with corresponding sensors and then processed correspondingly for the respective purposes. The nonmarking devices based upon a magnetic coding have the disadvantage they can are relatively simply reproduced or reset and thus afford only slight protection against falsification. In addition, such marking devices are sensitive to external magnetic fields and also tend to become weakened with respect to the strength of the magnetization with long term storage and under external influences (magnetic fields, temperature, etc) and can be completely lost.
It is the object of the invention to provide a marking device of the type described at the outset which has a high degree of security against falsification, is insensitive to external magnetic fields and cannot lose the coding.
This object is achieved in accordance with the invention in that the coding has at least one magnetic base layer and at least one magnetic coding layer which are so interrelated that over the extent of the base layer or the coding layer, they provide areas of nonparallel or antiparallel magnetic coupling.
The basic principle of the invention is thus to use two thin magnetic layers for the coding and thereby make use of the effect of magnetic interlayer coupling. This effect is known per se (P. Grxc3xcnberg et al., Layered Magnetic Structures: Evidence for Antiferromagnetic Coupling of Fe Layers across Cr Interlayers, Physical Review Letters, Volume 57, No. 19, 1986, P. 2442 to 2445) and is primarily used for magnetic field sensors (U.S. Pat. No. 5,159,513; EP-B-O 346 817) or also for magnetic plotting and writing heads, communications and shielding (DE-OS 28 27 429). A marking device based upon this effect has the advantage of a highly characteristic magnetic signal strongly different from the customary magnetic marking upon application of an external magnetic field. If the external magnetic field is strong enough, the nonparallel or antiparallel couplings can be interrupted which can give rise to a change in the magnetization of the corresponding local regions. If the external magnetic field is removed, the original magnetization with nonparallel or antiparallel couplings are reshaped. A conventional magnetic coding loses the recorded information upon magnetization to saturation since, in the remanent state, the individual regions assume the magnetization direction which is determined by the external magnetic field. This characteristic behavior of the nonparallel or antiparallel coupling regions affords the advantage that, upon producing the marking, there is predetermined locally structured magnetic pattern which serves for information storage. The magnetic behavior of this pattern in saturation and in remanence cannot be reset by pure parallel coupling magnetic coding and thus affords the aforementioned protection against falsification.
The above-described effect can also be utilized to reactivate magnetic coding which has been weakened as a result of prolonged storage time or even a lost magnetic coding, since the security device is interrupted by an external magnetic field. This opens the possibility of utilizing marking devices where magnetic coding has not previously been used, for example where an identification is only seldom required as with bank notes, parking places for vehicles, jewelry articles or the like. Since the coding is not damaged by magnetic fields and also can be reactivated at any time, there are no limits for the use of the marking device of the invention.
The marking device according to the invention has in addition the advantage that it is compatible with the conventional reading devices for magnetic data carriers, for example, AMR (anisotropic magnetoresistance) or GMR (giant magnetoresistance) sensors or magneto optical reading devices like magneto optical Kerr cell microscopes.
According to a feature of the invention, between the base layer and the coding layer at least in regions, for example, a nonpermanently magnetic intermediate layer can be disposed of such thickness that regions of nonparallel or antiparallel coupling are provided. By means of such an intermediate layer, for example of copper or chromium, utilizing usual magnetic materials for the base layer and the coding layer, like for example iron, cobalt, nickel or the like, a nonparallel or antiparallel orientation of the magnetization can be effected. Preferably the intermediate layer should be of such thickness that an antiparallel coupling is produced since that will magnetically structure the coding especially clearly. In addition, the intermediate layer should be provided only where the nonparallel coupling or an antiparallel coupling is to be effected. In this case as well, there is an especially clear magnetic structuring of the coding.
Alternatively, the coding layer can be comprised of a material which lies upon and thus intrinsically couples to the base layer in a nonparallel or antiparallel manner. Such materials are, for example, chromium as a monolayer on iron or rare earth metals like gadolinium or its alloys. With these materials the nonparallel coupling or antiparallel coupling is formed already upon direct application to the base layer and gives rise there locally to a change in the magnetic signal. This reverses itself under the effect of an external magnetic field up to saturation, i.e. in the regions where the coding layer is provided from the aforementioned material, the field strength is increased so that a certain magnetic structure obtains which can be detected by corresponding sensors. Upon removal of the external magnetic field, the original structure is again reestablished.
To produce the nonparallel or antiparallel magnetic coupling regions it suffices for the coding layer to cover only regions of the base layer. This is especially applicable to the case in which the coding layer is comprised of a material which already upon direct application to the base layer couples in a nonparallel or antiparallel manner therewith. An improved structuring of the coding can however be achieved when the base layer and the coding layer so interact that regions with nonparallel or antiparallel coupling and regions with parallel couplings are provided. This can be achieved above all in that the intermediate layer is limited to the regions where a nonparallel or antiparallel coupling is to be produced. The orientation and configuration of the base layer and coding layer can be such that, upon application of an external magnetic saturation field, a spatially nonuniform magnetization is provided which does not differ significantly from the spatial structure of the magnetization without the external magnetic field. Both magnetization impressions can be used for identification purposes on detection.
It is also advantageous for the base layer and the coding layer to be so configured that they have a uniform magnetization over the areas in a saturation magnetic field. The coding then disappears in the external magnetic field. This can be used, prior to the detection, to carry out a magnetization up to the saturation level. One can thus distinguish a coding which is produced by known methods and after through magnetization is destroyed and can no longer be reactivated and detected. By contrast thereto, the marking device according to the present invention enables, after removal of the magnetic field, the return of the original coding and its detection. The aforedescribed characteristic can be ensured most simply by providing the base layer and the coding layer of contrasting thicknesses while each generally is comprised of a single material. Under these conditions the clearest structuring can be. achieved when the base layer and the coding layer have the same magnetizations and the optional intermediate layer has such thickness that the coding layer in the region of the intermediate layer fully compensates the magnetization of the base layer. In the remaining regions, the magnetization is based upon parallel coupling.
The principles of the invention can be applied in varied form in that the base layer and/or the coding layer and/orxe2x80x94to the extent providedxe2x80x94the intermediate layers have regions composed of different materials and/or of different thicknesses. In this manner the coding can be of practically optional complexity, i.e. regions can be provided with a practically unlimited number of different magnetizations and thus even graduated values can be stored. As a consequence this ensures high reliability and effective protection against falsification.
It will be understood that the basic concept of the invention is effective not only with a single coding layer but also with a plurality of superimposed coding layers. As a result, this also enables a high complexity of the coding to be achieved. The coding layers which are used can be composed of different materials and can have different thicknesses. In the same way, the magnetic coupling of the coding layer or layers can be influenced also by the respective choice of the material and of the thicknesses of the intervening layer.